Screen-printing process for efficient n-PERT solar cells

Imec, within the partnership of EnergyVille, has announced that it has realised highly efficient n-PERT (Passivated Emitter and Rear Totally diffused) solar cells using an industry-compatible screen-printing process. Together with Jolywood, imec developed industry-compatible bifacial cells with an average front-side conversion efficiency of 21.9%. Based on this process, imec also demonstrated screen-printed monofacial n-PERT cells with a conversion efficiency up to 22.8%.

The new bifacial cells use narrow (~40μm) printed silver (Ag) fingers on the frontside and printed aluminum (Al) fingers on the rear, the latter making contact with the emitter. By using Al instead of AgAl for the rear contacts, the cost per cell is lowered with 0.01$/Wp.

Additionally, recombination in the contact area is reduced by avoiding the typical spiking of AgAl contacts. This allows to optimise the emitter independently from the contact firing, giving an additional boost to the efficiency.

Although the printed Al fingers on the rear side are inherently wider than the frontside fingers, the bifaciality of the cells is 88%, which is better than current state-of-the-art bifacial p-PERC cells. On a batch of M2-sized cells (area: 244.3cm²), an average conversion efficiency of 21.9% was demonstrated, with the best cell topping 22.1%.

These results were measured using an ISE CalLab calibrated reference cell, under standard test conditions using a AAA-class WACOM solar simulator. Used in bifacial operations under standard front illumination conditions in conjunction with an additional 0.15sun rear illumination, these cells can achieve an effective efficiency of 25%.

Imec also fabricated screen-printed monofacial nPERT cells with efficiencies up to 22.8%, which is a state-of-the-art result for an industry-compatible fabrication process.

To do so, the researchers transferred the screen-printing technique developed for bifacial cells to monofacial cell fabrication, with the cells’ backside fully covered by printed Al with dot openings in the dielectric to contact the B-doped emitter.

Loic Tous, project leader at imec/EnergyVille, said: “We are very pleased with the bifacial nPERT results obtained in close collaboration with Jolywood. These results fit very well with our roadmap towards 22.5% printed bifacial nPERT cells and 23.5% monofacial nPERT cells by the end of 2018."

"We are looking forward to continuing our developments with our colleagues at Jolywood and hope to contribute further to their expanding technological portfolio. Jolywood is aiming at developing the world’s best n-type cells and modules and we believe we are uniquely placed to help them reach that target.”

Dr. Zhifeng Liu, R&D director of Jolywood, said “Jolywood always strives to be at the forefront of the n-type industrial technological developments in PV industry. Especially we believe that, the excellent performance, reliability and bifacial gain of n-type bifacial solar cells can lead to much lower levelised cost of electricity (LCOE) and bring more benefits to our customers".

"We are very satisfied with the close collaboration with imec, one of the most renowned PV research institutes working on Si photovoltaics, and very delighted with the excellent progress in the development of the high efficiency bifacial n-PERT solar cells within our collaboration."

"Jolywood is always focused on cutting-edge technology innovation to further improve the cell efficiency and to lower the LCOE. With our strong commercialisation capabilities and experience, we look forward to transferring the achievements into commercial production.”

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